Later on when enabling legislation is presented in parliament, all government MPs will be instructed to vote for it. If a government MP looks like they might not vote in favor, well this is really unlikely, but if so they will come under immense pressure from many different directions. It won't happen.

If the government later changes to a different party - will that party change the treaty? Very unlikely in NZ, and that would mean renegotiating it with many other countries. It doesn't really happen. Once it is signed, it is signed.

The current government is not dumb enough to completely gut Pharmac and that will probably not happen but I would expect minor changes intended to slowly creep up on that sector over time.